Coating of solid particles may be advisable, for example, for metallic magnetic pigments in order to increase the corrosion resistance. For this purpose, the metallic solid particles are coated with a thin carbon-containing layer. The coating of active substances also represents a typical area of use. It is possible in this way, for example, to achieve delayed release of the active substance or improved stability.
It is possible by surface treatment of the solid particles to influence, for example, their wettability. Thus, the surface of colored pigments can be roughened by the action of oxygen, which facilitates dispersion of the colored pigments.
These processes must meet certain requirements, of which uniform coating or treatment of the surface of the solid particles should be mentioned first. In this respect as a rule only small variations are permissible, and the elaboration of the apparatus and process is correspondingly high in order to comply with this and similar objectives in the most economic way industrially.
In a known process, aluminum particles are coated with a carbon-containing layer in a fluidized bed into which a plasma torch projects ((T. Kojima et al., Journal De Physique IV, Colloque C2 (1991), "Development Of A Plasma Jetting Fluidized Bed Reactor", pp 424-436). In this case, argon is used as carrier gas and a mixture of methane and hydrogen is used as coating agent (page 430). The plasma is generated by a plasma torch which is operated with a constant voltage and projects directly into the fluidized bed. The plasma generation produces free radicals from the methane, which bring about a free-radical chain reaction to give larger organic molecules which in turn eventually form a crosslinked organic layer on the aluminum particles. However, owing to the direct action of the plasma torch on the fluidized bed in the process, very high coating temperatures are produced so that only thermally stable substrates such as metal or ceramic powders can be coated.
A process in which solid particles are coated in a low-pressure process with the aid of a cascade arc discharge device operated at constant voltage is also known (U.S. Pat. No. 4,948,485). The coating in this case is achieved by passing a stream of particles past the arc discharge device (column 6, lines 8 to 27). Although the reduction in pressure to below about 0.1 bar (column 2, lines 54 to 66) makes it possible to reduce the coating temperature, the particles can be exposed to the action of the arc discharge device for only a short time, which greatly restricts the possible uses of this process. In addition, in this process, it is extremely difficult to obtain uniform coating of the particles.
DE-A 36 17 428 describes a process in which the components are coated in a fluidized bed operated at atmospheric pressure, with, if appropriate, hydrogen being preheated by means of a plasma torch.